The present application claims priority to Japanese Application No. P11-305520 filed Oct. 27, 1999, and to Japanese application No. P2000-119589 filed Apr. 20, 2000, which applications are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a magnetization control method, information storage method, magnetic functional device and information storage device, which are suitable for application to a solid magnetic memory, for example.
2. Description of the Related Arts
Along with epoch-making diffusion of information communication apparatuses, especially, personal-use information apparatuses such as portable terminals, their constituent devices such as memory and logic are requested to have higher performances, such as higher integration, higher speed, lower power consumption, and so on. Especially, progression of nonvolatile memory toward higher densities and larger capacities is getting more and more important as complementary techniques with magnetic hard disks that are essentially difficult to progress reduction in size, increase the speed and decrease the power consumption because of the existence of movable members and other reasons.
Semiconductor flash memory and FeRAM (ferroelectric nonvolatile memory) are currently used in practice as nonvolatile memory, and various efforts are still continued toward higher performances. However, flash memory and FeRAM involve the following essential drawbacks derived from their fundamentals of operation, structures, and materials employed.
 less than Flash Memory greater than 
1. For writing, injection of hot electrons into floating gates is employed. However, injection efficiency is usually as low as 10xe2x88x926, and it needs a time in the order of ps for accumulating an electric charge sufficient for storage.
2. If an effort is made to increase the injection efficiency in (1) above, (for example, the use of Fowler-Nordheim tunneling injection), the device structure is inevitably complicated. This makes it difficult to realize a higher integration, and such a complicated structure causes a high cost.
3. Either hot electron injection or tunneling injection needs a high voltage (typically more than 10 V), and its power consumption is large. Additionally, an inverter is required for portable use, and this is disadvantageous for miniaturization.
4. A tunneling oxide film around floating gates deteriorates progressively with occurrences of writing. It results in inducing the leak current, promoting outflow of charges for storage, and degrading the reliability. Typical endurance is about 100,000 times.
 less than FeRAM greater than 
1. Ferroelectric element, which is a metal oxide, is easily affected by a reducing atmosphere that is indispensable for a silicon process, and it does not match such process.
2. Oxides, in general, need a process under a high temperature, which is a disadvantageous condition in microminiaturization of design rules. That is, this disturbs high-density integration. Although it has a device structure enabling integration as high as DRAM, or the like, it is generally considered that those factors limit the degree of integration to 10 Mb/inch2, maximum.
As nonvolatile memory not involving those drawbacks, magnetic memory called MRAM (magnetic random access memory) as disclosed by Wang et al., IEEE Trans. Magn. 33(1997), 4498, for example, has been recently brought into attention.
MRAM is a magnetic information storage device in which fine storage media of a magnetic body are regularly arranged, and wirings are provided to allow each of the storage media to be accessed to. Writing is performed by using a current-magnetic field generated by flowing a current in both a selection line (word line) and a read line (bit line) provided above the storage media and controlling magnetization of individual magnetic elements forming the storage media. Reading is performed by using a current-magnetic effect. Since MRAM has a simple structure, high integration thereof is easy, and because of its way of storage by rotation of a magnetic moment, its maximum endurance is large. Additionally, it does not need a high voltage, and needs almost no oxide difficult to make. Just after its proposal, a long access (read) time was a problem. Today, however, where a high output can be obtained by using GMR (giant magnetoresistance effect and TMR (tunneling magnetoresistance) effect, the problem of its access time has been improved significantly.
This MRAM, however, involves an essential problem in its writing method from the standpoint of more advanced integration. More specifically, as the wirings become thinner along with progressively higher integration, the critical value of the current that can be supplied to the write line decreases, and therefore, the magnetic field obtained becomes so small that the coercive force of the storage region must be lowered. This means that the reliability of the information storage device degrades. Further, since magnetic field, in general, cannot be converged unlike light or electron beams, its high integration may cause cross-talk. Thus, writing relying upon a current-magnetic field essentially involves those problems, which may become large drawbacks of MRAM.
Similarly to flash memory and FeRAM, those problems will be removed if magnetization can be controlled by mere electric stimulation, i.e., without using a current-magnetic field. A method according to this concept was already proposed, and there is a technique that uses a structure in which two ferromagnetic layers are separated by a semiconductor layer, as disclosed by Mattson et al., Phys. Rev. Lett. 71(1993) 185, for example.
This technique uses the fact that magnetic coupling between ferromagnetic layers depends on carrier concentration of a semiconductor layer interposed between them. In a multi-layered structure stacking such ferromagnetic/semiconductor/ferromagnetic layers, magnetic coupling between ferromagnetic layers can be changed from parallel to anti-parallel, for example, by controlling carrier concentration of the semiconductor layer as the intermediate layer. Therefore, if the coercive force in one of the ferromagnetic layers (fixed layer) is set large, then magnetization of the other ferromagnetic layer (movable layer) can be rotated relative to the fixed layer. Such method of rotating magnetization through an electric input is hopeful as a technique for realizing a compact all-solid-state component.
In the above-mentioned multi-layered structure stacking ferromagnetic/semi-conductor/ferromagnetic layers, magnetic interaction occurs indirectly between the ferromagnetic layers via the semiconductor layer. In this case, the semiconductor layer as the intermediate layer must be sufficiently thin because intensity of interaction between the ferromagnetic layers via the semiconductor layer attenuates exponentially with the thickness of the semiconductor layer.
Assume here that, for the purpose of obtaining an actual intensity of interaction, a coercive force of 100 Oe (Oersted) is given by a method like exchange biasing in a nickel-iron alloy having the thickness of 2 nm and the saturated magnetization of 12500 G (Gauss), for example. In order to give an energy equivalent to an energy necessary for reversing magnetization of this nickel-iron alloy by indirect interaction via the semiconductor layer, it is estimated by simple calculation that the exchange coupling constant J must be at least 0.02 mJ(Joule)/m2. Apparently, to ensure interaction not smaller than that value, thickness of the semiconductor layer as the intermediate layer must be 2.5 nm or less, as indicated by J. J. de Vries, Physical Review Letters 78(1997) 3023, for example. This is a term imposed to the semiconductor layer as the intermediate layer to provide a practical device.
On the other hand, in order to control magnetic coupling between the ferromagnetic layers by controlling carrier concentration of the semiconductor layer as the intermediate layer, an electrode has to be attached to the semiconductor layer in any appropriate manner for application of a voltage or injection of a current. Additionally, the device structure including this electrode must be optimized to effectively control carrier concentration of the semiconductor region between those two ferromagnetic regions (that serve as storage media). However, since thickness of the semiconductor layer must be 2.5 nm or less as explained above, it is difficult to actually fabricate such a device with existing fine processing techniques. Even if such a device can be made actually, since the semiconductor layer having that order of thickness is considered to form a substantial insulating barrier, it is extremely difficult to reflect modulation of the carrier concentration to the control of magnetic coupling.
It is therefore an object of the invention to provide a magnetization control method capable of readily controlling magnetization without using a magnetic field, information storage method, magnetic functional device and information storage device using same.
According to the first aspect of the invention, there is provided a magnetization control method comprising:
placing a potential barrier region in direct or indirect contact with a region containing a magnetic element; and
controlling magnetization of the region containing a magnetic body by modulating the potential barrier of the potential barrier region.
According to the second aspect of the invention, there is provided an information storage method comprising:
placing a potential barrier region in direct or indirect contact with a region containing a magnetic element;
controlling magnetization of the region containing a magnetic body by modulating the potential barrier of the potential barrier region; and
storage of information being effected by using at least one of magnetization of the region containing a magnetic element.
According to the third aspect of the invention, there is provided a magnetic functional device comprising:
a structure including a potential barrier region placed in direct or indirect contact with a region containing a magnetic element,
magnetization of the region containing a magnetic body being controlled by modulating the potential barrier of the potential barrier region.
According to the fourth aspect of the invention, there is provided an information storage device comprising:
a structure including a potential barrier region placed in direct or indirect contact with a region containing a magnetic element,
magnetization of the region containing a magnetic body being controlled by modulating the potential barrier of the potential barrier region; and
storage of information being effected by using at least one of magnetization of the region containing a magnetic element.
Typically, in the present invention, the region containing the magnetic body has a multi-layered structure in which a plurality of ferromagnetic layers are stacked and separated by an intermediate layer; the potential barrier region is a potential barrier layer located outside the multi-layered structure in the stacking direction; and relative arrangement of magnetization of the ferromagnetic layers in the region containing the magnetic body is controlled by controlling height and/or width of the potential barrier of the potential barrier layer. More specifically, for example, the region containing the magnetic body has a multi-layered structure in which a plurality of ferromagnetic metal layers are separated by a non-magnetic metal intermediate layer; the potential barrier region is a potential barrier layer formed along the interface with the semiconductor layer located outside the multi-layered structure in the stacking direction; and relative arrangement of magnetization of the ferromagnetic metal layers inside the region containing the magnetic body is controlled by controlling height and/or width of the potential barrier of the potential barrier layer by means of application of an electric field to the potential barrier layer. Alternatively, the region containing the magnetic body has a multi-layered structure in which a plurality of ferromagnetic layers are separated by a non-magnetic metal intermediate layer; the potential barrier region is a potential barrier layer consisting of an insulating layer located outside the multi-layered structure in the stacking direction; and relative arrangement of magnetization of the ferromagnetic metal layers inside the region containing the magnetic body is controlled by controlling height and/or width of the potential barrier of the potential barrier layer by means of application of an electric field to the potential barrier layer.
In the present invention, the region containing the magnetic body may be a single ferromagnetic layer; the potential barrier region may be a potential barrier layer arranged to contact the ferromagnetic layer via a non-magnetic layer having a thickness not thinner than a one atom layer; and direction of magnetization of the ferromagnetic layer may be controlled by controlling height and/or width of the potential barrier of the potential barrier layer. More specifically, for example, the region containing the magnetic body is a single ferromagnetic layer; the potential barrier region is a potential barrier layer formed along the interface with the semiconductor layer located in contact with the non-magnetic layer having a thickness not less than one atom layer; and direction of magnetization of the ferromagnetic layer is controlled by controlling the height and/or width of the potential barrier of the potential barrier layer by means of application of an electric field to the potential barrier layer. Alternatively, the region containing the magnetic body is a single ferromagnetic layer; the potential barrier region is a potential barrier layer consisting of an insulating layer located in contact with the ferromagnetic layer via a non-magnetic layer having a thickness not thinner than one atom layer; and direction of magnetization of the ferromagnetic layer is controlled by controlling the height and/or width of the potential barrier of the potential barrier layer by means of application of an electric field to the potential barrier layer.
In the present invention, usable as the magnetic functional element is a switching element similar to a field effect transistor (FET) using the phenomenon that the magnetic resistance varies with a voltage applied, for example. By using one or more switching elements, various circuits, including a logic circuit, for example, can be made.
In the present invention having the above-summarized-configuration, by modulation of the potential barrier of the potential barrier region arranged in direct or indirect contact with the region containing the magnetic element, quantum-mechanical reflectance of electrons along the interface between the region containing the magnetic body and the potential barrier region can be modulated. And, since interference of electron waves takes part in the magnetic interaction inside the region containing the magnetic element, magnetization of the region containing the magnetic body can be controlled through modulation of the quantum-mechanical reflectance. In this case, since the potential barrier region can be located outside the region containing the magnetic element, the electrode for input such as application of a voltage can be arranged easily. Therefore, magnetization of the region containing the magnetic body can be readily controlled without applying a magnetic field, and information can be stored by using at least one of magnetization of the region containing the magnetic element, for example.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.